The present invention relates to an integrated high voltage resistive structure on a semiconductive substrate, and more specifically to a serpentine integrated resistive structure on a semiconductive substrate having a first type of conductivity opposite to that of the semiconductor substrate.
Embodiments of the invention relate in particular, but not exclusively to a resistive structure at high voltage to be integrated on a semiconductive substrate together with power devices and the following description is made with reference to this field of application with the sole objective of simplifying its disclosure. Discussion of steps or processes well know to those skilled in the art has been abbreviated or eliminated for brevity.
As is well known, high voltage resistive structures which are integrated on a semiconductive substrate find ample use in the application field for power devices formed as integrated circuit, for example VIPower devices.
VIPower devices integrate on the same chip a region on which the power devices are formed (power region) and a region on which signal devices are formed (signal region). In some applications, it is necessary to arrange, inside the signal region, a division of the substrate voltage. This can be provided by using a resistive structure connected between the substrate and a control region of the signal device. This resistive structure will therefore be subjected to the substrate voltage Vs which as is well known, in devices of the VIPower type, can reach elevated values of up to 2 KV, hence the term resistive structure or high voltage resistance HV.
In FIGS. 1 and 2 the electrical diagrams of two examples of possible applications of the high voltage resistive structures are shown.
In FIG. 1 for example, a first circuit structure C1 is shown comprising a bipolar component Q1 of the NPN type connected in series on the emitter region to a first terminal of the resistance R1. A Zener diode D1 is connected, in inverted polarization, between the base terminal of the component Q1 and a second terminal of the resistance R1. A high voltage resistance RHV is connected between the collector region and the base region of the component Q1.
When a current I1 flows through the resistance RHV, the component Q1 switches on and drives a low voltage circuitry BT connected to an emitter region of the component Q1. The current which flows through the resistance RHV obviously depends on the substrate voltage Vs and on the value of the resistance itself.
In FIG. 2, a second circuit structure C2 is shown comprising two circuit branches 1a and 2a having a common node A. The first branch 1a comprises a Zener diode chain D2, D3 and D1 connected to the base region of a first bipolar component Q2, which is polarized by a resistance R2. The second branch 2a comprises a resistance R3 connected in series to the emitter region of a second bipolar component Q3, which is controlled by a battery Vb. A high voltage resistance RHV is then connected to node A.
In this configuration the voltage value on node A can be used as a reference value for permitting conduction on branch 1 or on branch 2, depending on the value of Vz of the Zener chain, of the Vb battery voltage as well as from the other components present in the circuitry. In this case, the resistance RHv is used simply as a voltage divider.
In both examples, the voltage of substrate Vs applied to the resistance RHV, as said before, can reach elevated values. The voltage divider used as a driver signal for the linear region (circuit C2), and also the current which flows through the resistance HV (circuit C1), assumes values which must be comparable and therefore not above the maximum voltage of the well inside which the signal circuitry is integrated, and therefore of the maximum current foreseen for a determined circuit structure. This means that the resistance RHV must have a resistive value such as to permit the division or the current required by the driving circuitry as foreseen by the circuit structure used.
This resistance value can also be in the order of some Mxcexa9 and in any case not less than a few tens of Kxcexa9.
A first known technical solution for the formation of resistive structures with high resistive values foresees forming doped regions having a high resistivity on a semiconductive substrate.
Though advantageous in many respects, this first solution has various problems, in particular, when forming regions of high resistivity, fairly high area dimensions are required for the die.
Another solution of the prior art foresees the formation of long resistive structures which, according to the area used, minimize the dimensions of silicon occupied thanks to a particular layout.
One layout embodiment according to the prior art is shown in FIG. 3. In particular, in a substrate 1xe2x80x2 of N type a serpentine region 2xe2x80x2 of P type is formed. This type of layout, nevertheless, cannot be used for the resistive structure at high voltage, because it would occupy a fairly large area of silicon. This is due to the size of the depletion region 3xe2x80x2, outlined in FIGS. 3 and 4, that is inversely proportional to the concentration of dopant (and therefore directly proportional to the resistive value), during inverted polarization of a portion of doped silicon and therefore the size of this depletion region is very important in the resistive structures RHV.
Even if the high voltage resistive structures can be integrated by using the more resistive layers used in the technology, VIPower devices capable of supporting elevated voltages necessarily have an elevated resistivity of the substrate, in varying degrees of size bigger than the more resistive layers available with current technological processes. This means that layouts which tend to optimize area availability of silicon on chips such as that of FIG. 3, have the problem of pinch-off phenomenon.
In particular, the depletion regions of two or more parallel branches of the resistive structure come into contact, as illustrated on the right side of FIG. 4, with subsequent alterations in the values of the resistive structure itself and therefore of the functioning of the circuitry of which it is a part.
In order to overcome this problem, it is necessary in the design phase of the layout for the high voltage resistive structure that the distance between the various branches of the serpentine resistive structure which face each other in parallel, should be more than the total of the widths of the depletion regions which belong to each branch. This means that the branches of the resistive structure subjected to a high voltage must be set apart according to the drop in voltage on the resistive structure itself.
As a consequence of this, the layout in FIG. 4, in the case of a high voltage resistance structure, takes the form shown in FIG. 5 with considerable dimensions of silicon areas.
Furthermore, the high voltages placed on the resistive structure, would require border structures, capable of protecting the more pressing regions against premature breakdowns from the high voltages. Metal field plates or rings with a high resistive structure are used for example in this case, which anyway tend to further increase the area of silicon occupied.
In order to reduced the lateral depletion region between the various branches of the resistive structure, a known technique enriches the layer intended for integration of the resistive structure itself. Nevertheless this solution reduces the capability of the device to hold the voltage, in that in order to obtain a reduction of the widening of the depletion region it would be necessary to have a concentration of dopant in the surface region which would be very high.
The same considerations made above can also be repeated in the case in which the high voltage resistive structure is integrated around the region at high voltage which surrounds the device. In this way, especially if the device occupies a large area, a length of the resistive structure equal to a fraction of the entire perimeter of the device or at most equal to one or two perimeters permit the formation of the resistive structure desired.
In this case, in fact, the distances to be kept in mind in the design phase, involve the distances between the branches of the resistive structure itself and the well in which the power device is formed.
Embodiments of the present invention form a serpentine resistive structure integrated on a semiconductive substrate, having structural and functional features such as to allow high voltage to be sustained without incurring the pinch-off phenomenon between the parallel branches of the serpentine, overcoming the limitations and drawbacks which limit now the resistive structures formed according to the prior art.
The resistive structure is formed with integrated serpentine on a semiconductive substrate, in which, between at least two parallel portions of the serpentine, insulation regions are formed.
The characteristics and advantages of the device according to these embodiments are presented in the description, given hereinbelow, of an example of embodiment given as an example and not limiting with reference to the attached designs.